Doomsday Candle is part of the warlock epic mount questline, in which you acquire a Dreadsteed. The Dreadsteed was previously only available to a Warlock who completed a long quest chain (of which this is a part), but was changed in Patch 3.0.3 to be available to all warlocks at level 61 from the mount trainer.

Doomsday Candle

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A Doomsday Candle stores fire and directs it at those who would disrupt your ritual. When creating your Circle of Greater Summoning, if your candle is lit then any wayward demons will be burned by its flames!

Containing that fire is not easy, and requires the essence within the scales of a black dragon. Acquire those scales from a skinner and bring them to Gorzeeki Wildeyes.

Ah, very good. I'll need these to contain the fires of the candle. Now that I have the scales, I'll make the candle and hold onto it until you're ready to begin your ritual for the Circle of Greater Summoning.